Influence of the COVID-19 lockdown on Spanish professional soccer teams’ external demands according to their on-field ranking

The main objective of this study was to analyse the changes in external demand parameters (e.g., total distance, high-speed running distance, accelerations/decelerations) in Spanish professional soccer teams after the COVID-19 lockdown considering their on-field ranking (i.e., teams whose ranking worsened after the COVID-19 lockdown [WRS] vs. teams that improved their ranking after the COVID-19 lockdown [IMP]). A total of 23,527 individual match observations were collected on players competing during the 2019/20 season in the First Spanish Professional soccer League (LaLiga). Goalkeepers and players who participated for less than 10 minutes in each match were excluded. Relative total distance (TD/min), distance covered at 21–24 km · h-1 (HIRD/min) and > 24 km · h-1 per minute (VHIRD/min), high metabolic load distance (HMLD), and the number of accelerations (3 m/s2) and decelerations (< 3 m/s2) performed were analysed by the ChryonHego video-tracking system. These variables were analysed during two differentiated periods, before the COVID-19 lockdown (i.e., 27 matches) and after the COVID-19 lockdown (i.e., 11 matches), and teams were classified into two groups according to their ranking (i.e., WRS vs. IMP). R-Studio was employed for data analysis and a mixed linear model was conducted. A decrease in external demands in all teams after the COVID-19 lockdown was observed, and this decrease was greater in WRS. These results suggest that, after an inactive period (i.e., the COVID-19 lockdown), teams that return with better physical performance, mainly related to high-intensity actions, have more possibilities of improving their final qualifying position.


INTRODUCTION
Second Division [19]. In the other hand, regarding match outcome, international female soccer players registered higher total distance and performed a higher number of high-intensity running efforts when winning in comparison with drawing or losing [20]. In contrast, Castellano et al. [21] found that when the result was adverse, the distances covered were greater. Deeper analysis confirmed that total distance and distance covered at more than 21 km/h -1 by central midfielders, wide midfielders, and forwards significantly increased while winning (p < 0.05 [22]). Thus, knowing the changes in performance after the COVID-19 lockdown differentiating between successful teams and unsuccessful teams, it will allow us to establish specific training strategies. Therefore, the main objective of this study was to analyse the changes in external demand parameters (e.g., total distance, highspeed running or acceleration/deceleration) in Spanish professional soccer teams after the COVID-19 lockdown considering their on-field ranking (i.e., teams whose ranking worsened after the COVID-19 lockdown [WRS] vs. teams that improved their ranking after the COVID-19 lockdown [IMP]). Based on previous studies [5,17], we hypothesized that external demand parameters would decrease in both groups (i.e., WRS and IMP), with larger decreases in the WRS group.

Study design
A retrospective, quasi-experimental longitudinal design was applied to compare the changes in Spanish professional soccer players' external demand parameters after the COVID-19 lockdown between WRS and IMP teams. Two differentiated periods of the 2019/20 season were analysed: the first period (pre-lockdown), from the 1 st to the 27 th match day (matches played over 30 weeks), and the second period (post-lockdown) from the 28 th to the 38 th match day (matches played over 5 weeks) ( Figure 1). To aid in determining the effect of the lockdown on soccer external demands, a pairwise performance [14,15]. Related to this, 5 substitutions were allowed in post-quarantine matches [2], which could influence the external demands related to matches [16].
Based on the aforementioned special situation, it seems pertinent to analyse the changes in match physical demands imposed on soccer players after the COVID-19 lockdown compared to their prior demands. Brito De Souza et al. [5] compared the match running performance in teams competing in LaLiga before (2018/19 season) and after the lockdown due to COVID-19 (2019/20 season) and observed that all variables studied were maintained (i.e., total distance and distance covered below 14 km · h -1 ; at 14-20.9 km · h -1 ; at 21-23.9 km · h -1 ; and above 24 km · h -1 ). However, when only the last 11 matches were analysed (i.e., matches after the COVID-19 lockdown), a significant reduction in the distance covered at 14-20.9 km · h -1 was observed during the 2019/20 season ing other countries, in the Italian Serie A competition, significant decreases in total distance and distance covered at high-intensity were observed (p < 0.001) [18]. However, these studies considered all teams together, so further studies based on performance variables (e.g., initial and final ranking) to classify and to compare soccer teams must be carried out.
Previous research has established that the top-ranked teams of the First Spanish professional soccer league covered significantly greater distance than the other teams belonging to the First and

COVID-19 Effects on External Demands in Soccer
comparison of match physical demands was performed between these two periods. Matches rather than total days were prioritized because the fixture schedule in the first period is less congested, and if the same number of days had been taken, the pre-lockdown period would have had fewer match days because matches took place once a week, and in the post-lockdown period, matches took place twice a week.

Participants
The sample was composed of 23,527 individual match observations of 925 professional soccer players belonging to soccer teams that Goalkeepers [18] and all players who participated in matches (starters and non-starters) but played less than 10 minutes were excluded because it was observed that average values obtained for these players were higher than the team average. A total of 6,341 recordings were excluded due to inclusion criteria, issues related to repeated signal loss by the system, or adverse weather conditions during the match that hindered accurate data collection. Data were provided to the authors by LaLiga, which had informed all participants through its protocols. All data were anonymized according to the

Variables
Match physical demands. The following variables were recorded in m · min -1 by Mediacoach at different speed ranges: relative total distance covered by players per minute (TD/min), distance covered by players in the range 21-24 km · h −1 (i.e., HIRD/min = high-intensity running distance per minute), and distance covered at more than 24 km · h -1 (i.e., VHIRD/min = very high-intensity running distance per minute). Additionally, we collected: high metabolic load distance (i.e., HMLD = distance covered at a speed greater than 5.5 m/s and while accelerating/decelerating at a magnitude of 2 m/s 2 or above) and the number of accelerations (3 m/s 2 ) and decelerations (< 3 m/s 2 ) performed and maintained for at least half of a second. These speed zones and accelerometry-based measures have been used in previous soccer studies [23,24].

Procedure
All the study variables were collected through the optical tracking system ChyronHego (TRACAB, New York, US), which is composed of 8 super 4K high dynamic range cameras based on a positioning system (Tracab -ChyronHego VTS). This system records from several angles and analyses X and Y positions for each player, providing real-time three-dimensional tracking (acquisition frequency = 25 Hz).
The validity and reliability of the Tracab video tracking system have been analysed, reporting average measurement errors of 2% for the total distance covered [25]. In addition, recent studies have tested the agreement between the Mediacoach system and GPS devices for assessment of physical demands. Specifically, the intraclass correlation coefficients (ICC) were higher than 0.90 [23]. This agreement has been checked in some studies [24].

Statistical analysis
Data were analysed using R-studio for Windows [26]. A linear mixed model (LMM) analysis was carried out for each of the eight models using the MIXED procedure. LMM allows analysis of data with a hierarchical structure in nested units and has demonstrated its ability to cope with unbalanced and repeated-measures data [27]. For example, the distance covered in matches is nested for players across time (i.e., each player has a record for any match played). To determine the adequacy of this statistical procedure, we first calculated the levels of within-person variance for each player by constructing unconditional null models. These unconditional models allowed us to calculate the intraclass correlation coefficient (ICC), which showed values greater than 10%, indicating the existence of variability in the data and justifying this analysis approach [28]. Subsequently, some separate random intercept models were constructed for each outcome measure, with periods included as fixed effects. In this way, we compared the values of the variables that the players obtained in the matches before the COVID-19 lockdown with the values of each of the following matches after the COVID-19 lockdown. Mean difference (Δ%) = (mean 1 -mean 2) × 100/mean 2) and Cohen's effect sizes (ES) [29] were also calculated to quantify the magnitude of difference for all pairwise comparisons using the following thresholds for interpretation: trivial, < 0.20; small, 0.20-0.59; moderate, 0.60-1.19; large, 1.20-1.99; very large, 2.00-3.99; and extremely large, > 4.00 [30]. Lastly, the difference between both WRS and IMP teams before and after the COVID-19 lockdown was analysed by the interaction.

RESULTS
Differences in external demand parameters considering periods (i.e., pre-and post-COVID-19 lockdown) and groups (i.e., WRS and IMP) are presented in Table 1

DISCUSSION
The main objective of this study was to analyse the changes in external demand parameters (e.g., total distance, high-speed running distance or accelerations/decelerations) in Spanish professional soccer teams after the COVID-19 lockdown, considering their on-field ranking (i.e., WRS vs. IMP). Although two previous studies [5,17] have analysed the changes in physical performance after the COVID-19 lockdown in a similar sample, this is the first study that Notes. SD = Standard Deviation; ES = Effect Size; TD/min. = Total distance covered per minute; HIRD/min. = High intensity running distance covered per minute; VHIRD/min. = Very high intensity running distance covered per minute; HMLD = high metabolic load distance; WRS = teams that worsened their qualifying position after COVID-19 outbreak; IMP = teams that improved their qualifying position after COVID-19 outbreak. * p < .05; ** p < .01; *** p < .001.
absence of friendly matches in this preparation, the congested schedule after the COVID-19 lockdown [2], and even by the fact of playing without an audience [17]. Contrary to our results, Brito de Souza et al. [5] reported no differences in high-intensity running distances after returning to competition, although these authors compared running patterns of the 2019/20 season entirely versus the 2018/19 season, which was used as a control. On the other hand, our results revealed that the number of accelerations and decelerations increased after the COVID-19 lockdown, although only in the WRS, similar to the results obtained by García-Aliaga et al. [17]. Finally, although the WRS group showed a significant decrease in VHIRD, it was not decreased in the IMP group, which could explain the improvement observed in the on-field ranking of these teams after the COVID-19 lockdown [33].
Due to the proven relationship between the on-field ranking and high-intensity running actions, especially those completed above 21 km · h -1 and with ball possession [33], analysing the changes in external demand parameters after the return to competition in professional soccer players is warranted. In this regard, the betweengroup comparison of the physical performance related to pre-lockdown matches only revealed differences in high-speed running in favour of IMP, and in accelerations for WRS. However, these differences increased in the post-lockdown match analysis, as IMP presented better performance in the TD, HIRD, VHIRD, and HMLD variables compared to WRS, without differences in the number of accelerations and decelerations. These results could explain the relevance of these actions in the professional soccer teams' on-field ranking [33]. Nevertheless, and with the aim of delving into the study of the influence of physical performance on the professional soccer teams' on-field ranking, it seems pertinent not only to focus on values relative to a specific moment but also to take into consideration the change in match physical demands after returning to competition compared to previous values. Although both groups experienced a decrease in external demand values, IMP suffered a smaller reduction than WRS in the TD, HIRD, VHIRD, and HMLD variables, which seems to confirm that these teams improved their on-field ranking after the COVID-19 lockdown matches, because previous studies have shown that successful soccer teams cover greater distances at high-intensity running compared to less successful ones [34,35]. Therefore, and for future similar situations, it would be appropriate to apply strategies such as increasing the group training period and scheduling friendly matches before returning to competition, which could facilitate reaching levels of physical performance similar to the pre-lockdown period in order to attenuate the decrease of the onfield ranking.
Although this investigation attempts to respond to an exceptional situation of great importance today, it has some limitations that must be taken into consideration. Firstly, only physical variables were considered, even though the detraining caused by the confinement could also have influenced tactical-technical, physiological, or psychological variables. Neither the tactical changes provided by new coaches nor the playing style which could influence match running performance were considered.
Secondly, only players from the First Spanish professional soccer league were involved in this study, so it is difficult to extrapolate these results to players from other European leagues. Finally, there is no information on the work carried out in confinement by each player/team, which could influence their physical fitness after returning to competition.

CONCLUSIONS
The results of this study confirm that Spanish professional players suffered a decrease in their external demand variables after the COVID-19 lockdown. In addition, this decrease was greater in the WRS after the COVID-19 lockdown. Finally, these teams presented lower external values in comparison to the IMP when only postlockdown matches were analysed.

Practical applications
From a practical approach, the findings obtained in this study are of interest for possible new confinement, suggesting the need for a longer specific preparation time and to complete friendly matches during this preparation so that players return to the competition in suitable conditions, favouring their on-field performance and reducing the risk of injury.

Conflict of interests
The authors declared no conflict of interests regarding the publication of this manuscript.

Funding details
Financial support provided by the European Regional Development Fund (ERDF), the Government of Extremadura (Department of Economy and Infrastructure) and LaLiga Research and Analysis Sections.